A method of measuring the contents of a search volume using electromagnetic energy is described in U.S. Pat. No. 5,920,285. Individual transmit elements of a transmit array are actuated in turn in order to interrogate the search volume. Reflected signals are recorded, and appropriate phase or time shifts are inserted to simulate, post reception, the shifts that would have occurred if either or both of the transmit and receive array had been focused on the cell using phased array beam steering techniques.
Another method of measuring the contents of a search volume is described in WO 2006/085052 A2. The method includes the steps of: energising one or more transmitters so as to transmit electromagnetic wave energy into the search volume; detecting the effect of the search volume on the passage of the electromagnetic wave energy by recording two or more signals at one or more receivers, each signal being associated with a different transmitter/receiver pair; pre-processing the signals to generate two or more pre-processed signals, each pre-processed signal being associated with a different transmitter/receiver pair; aligning the pre-processed signals in order to generate two or more aligned signals which are synthetically focused on a desired voxel in the search volume, each aligned signal being associated with a different transmitter/receiver pair; and summing the aligned signals to generate an output which is indicative of the internal structure of the search volume at the location of the desired voxel.
Various methods of removing skin surface artefacts are described in WO 2006/085052 A2. However these methods are not effective in removing clutter caused by other effects. These effects include single and multiple reflections from parts of the antenna, its feed, the array structure and the body.